Within the parameters of how your software can and cannot be set up, as Dimples says, if you want to raise more funds the probability is best to split them. Would someone be able to win both? Odds would be way against them. And of course you can't lie to someone.
Feelings (objective) vs Facts (subjective). As far as a set or split...
Do you care whether the winner would prefer a set or single? You cannot guess what someone else may or may not desire. The stated goal is to generate funds.
Do you care whether they are split? If you can't bear the thought of them being split or the chance the winner would want both, don't split.
Or the goal is just the most money generated for your charity? Probability is split, more tickets sold, more money generated.
Or within the parameters of your software, split and devise a rule for the subjective side of the equation to cover split vs non-split, e.g., first picked has the option of purchasing the second picture for X dollars; if they choose not to purchase, second draw for second picture.
Caveat 2: I worked security for our state lottery for years. 
Hybrid Mode
